AF readings at launch

[Chargermal]

Wondering what I should be seeing on my analogue face AF gauge when Plant the pedal to the floor?

By this I mean, should the meter swing into the rich zone as the pump shots take effect, or is it better if it drops just a little below stoich?

What Im trying to figure is how do you measure the ideal pump shot, instead of just setting on the richest position and chewing excess fuel for no reason.

I have a wide band Aem gauge.., dual quad AFB 500 induction.......just need to know what I should be seeing?

[user-612937456]

rich is good lean is bad when you are at full throttle. it is a balancing act under full acceleration you want it slightly rich at higher rpms and much richer under launch rpm's adjust your pump for the hardest launch power don't look at the AF for this give the engine what it likes

[Greenlight]

You want the engine to stay in the A/F ratio "zone" that it makes the most power in. That is usually somewhere in the 12.5 and 13.5 range for gasoline.

However, it's not always that easy to do.

The graphs below show two examples of vehicle launches. One with a transbrake (carb @ WOT on launch) and one vehicle that leaves on the brake. I'm not saying either one of these is ideal, but they are typical.

[Chargermal]

Guys that is fantastic advice - thank you very much.

My gauge is showing 17+ in the first second or so..... Clearly it needs more shot and better air speed.

[RednGold86Z]

Guys that is fantastic advice - thank you very much.

My gauge is showing 17+ in the first second or so..... Clearly it needs more shot and better air speed.

If you're stomping it, a carb will probably need to the pump shot to send it pretty rich - like high 10's-11's or 12's if you've got it dialed in and don't need it to work great when cold. If you're using EFI and have really worked on this to death, you can get a near ideal trace - but it's darn hard.

At 17+ it's gotta be feeling weak, or is it even backfiring or bucking? If you ease into a little bit slower, how is it? Can you lug it down with a manual trans? If your jetting/rods are wrong or the air valve opens too quickly, it could look like it's just the pump shot. If you band-aid the real problem, you'll go in circles.

[Chargermal]

It's easier to answer those questions with a picture - you can see that intake plenum on the Indy Modman intake is HUGE - that is all one big hollow space under those carbs!

- suffice to say it feels weak, but its not bogging or bucking - I believe its a combination of plenum volume, (pump shot) and air speed - I'll be experimenting with turtles...but needed to know what to look for on my AF gauge as far as whats working and what isn't.

[BigBlocksOnTop2]

Put some big pump shooters in it and get ready for a harder launch!

[Greenlight]

Here is a graph showing a 1st gear launch from a NHRA Stocker with a Hemi and 2 Carter (Edelbrock) carburetors. You should be able to easily get rid of the lean spike at launch.

[F-BIRD'88]

The timing curve graph from your previous posts looks all out of wack for this engine.
Explain.
Is this using vacuum advance? Or EFI system MAP controled advance?

Or?

What camshaft is in this motor?
Carb model? Jetting?
Timing curve?
Converter stall?
Stage rpm?

The afr gauge does not read AFR. IT reads oxygen in the exhaust.
Engine misfire reads as a lean afr on the gauge.
The afr gauge does not know which cylinder(s)
Is too lean, or which cylinder(s) is misfiring on launch.

The carb idle circuit and off idle circuit afr balance
Is critical. What is the afr at various small throttle openings.
Off idle low speed driving. Part throttle driving.
Ya it matters.

Edelbrock carbs in general and cammed up motors need
A lot lot lot of initial spark advance.
Generally a fixed " locked" spark adance curve is best
For drag racing. What ever it needs for WOT max power
Spark advance is what it needs at idle.
Lock the spark advance. Disable any all vacuum or map
Controled advance. ( for now).

If it needs 38° timing at full power. Thats what it needs at idle, and launch.
Lock out the timing. This is where you start.

Open headers without collector extensions and or leaking exhaust gaskets will
Throw off the afr gauge and can even throw off the engine afr at launch.
Free air reversion up the open header or ex leak, into the combustion chamber.
This extra air makes the afr lean and this nakes the engine misfire spotter bog on launch.
Bigger carb shooters won't fix this. Correct exhaust without leaks does.
The tuning of the sec air door action matters. We'll get to that.

Correct the timing, idle, off idle transition, and primary part throttle cruise rpm AFR.

If this is a serious racey engine and you more power you should have used two 750's or two 800's on it.

500's will need more tuning. And will limit max power.
YA IT MATTERS.

Why did you not just get another matching 800 avs eddy carb for the one you already had?

They are not too big and the sec air door action is easily adjustable.

[F-BIRD'88]

Basic spark plug gap matters. If you are using
Some magic, voodoo spark plug gap.
Return to normal .035"- .040" gap.
Be sure the ground strap end does not go
Past the center post. Trim it if nessesary.
File Smooth any sharp edges you create.

[F-BIRD'88]

Do you have a dyno test , from a engine dyno, not a chassis dyno.
Post it.
Converter stall speed?

[cuslog]

Mine is a footbrake car. I torque it against the brakes at about 2500 RPM then mash the throttle and release the brakes.
My AFR is low 12's against the brakes, then spike downwards to about 9.5 AFR for a fraction of a second with the pump shot.
The pump shot, start to finish is roughly one second, then it's back to low 12's, high 11's.

[cuslog]

Here's a screen shot of the log, pasted into Excel.
Black line is RPM, purple is AFR
This is 2X 850 Mighty Demons on a 6-71 Blower on a 540 BBC.
Squirters are #31

[Chargermal]

My responses in bold -

The timing curve graph from your previous posts looks all out of wack for this engine.
Explain.
Is this using vacuum advance? Or EFI system MAP controled advance?

The timing curve is what was recommended by a couple of established G3 hemi tuners - remember, this is no old school iron head motor - they dont need a lot of timing. It uses MAP advance

Or?

What camshaft is in this motor? - 218@.50
Carb model? Jetting? - Eddy 500s
Timing curve? - idle 18deg, all in by 4000, total 27 - It made no more hp with 30 total
Converter stall? - 3800 - 60ft is no better with a 3000 launch

There is no " misfire" - just a weak early throttle response.


What is the afr at various small throttle openings. - 14 at idle, 15 at cruise at 3000, 12.8 at WOT after initial lean out.

Efelbrock carbs in general and cammed up motors need a lot of initial advance - Motor is not cammed up at 218/227@.50 HR

If it needs 38° timing at full power. Thats what it needs at idle, and launch.
Lock out the timing. This is where you start. - It only wants 27 deg - think LS...not LA

Open headers without collector extensions and or leaking exhaust gaskets will
Throw off the afr gauge and can even throw off the engine afr at launch. N/A
Free air reversion up the open header or ex leak, into the combustion chamber.
This extra air makes the afr lean and this nakes the engine misfire spotter bog on launch.
Bigger carb shooters won't fix this. Correct exhaust without leaks does.
The tuning of the sec air door action matters. We'll get to that.

N/A - No leaks

Correct the timing, idle, off idle transition, and primary part throttle cruise rpm AFR - N/A - Its all fine.

If this is a serious racey engine and you more power you should have used two 750's or two 800's on it. - Its only 370 cubes - and almost factory


They are not too big and the sec air door action is easily adjustable - There are no issues with carb tuning outside of the stated lean out at wot launch. The motor ran 2.5 mph faster with the dual quads than the Pro system Holley or single 800 AVS - This also tells me that fuel distribution is the root cause...

[Chargermal]

Basic spark plug gap matters. If you are using
Some magic, voodoo spark plug gap.
Return to normal .035"- .040" gap.
Be sure the ground strap end does not go
Past the center post. Trim it if nessesary.
File Smooth any sharp edges you create.

Thanks - Ill check that - does this apply to iridium plugs?